It is known to monitor a structure by detecting reflection of ultrasound waves from within the structure. This makes it possible to detect defects such as cracks in the structure, because defects give rise to ultrasound wave reflection. In particular, defects in a wall of the structure may be detected. As used herein, a wall is a part of the structure that extends from a surface of the structure to a limited distance below the surface, so that it supports ultrasound waves that propagate in a lateral direction, that is a direction parallel to the surface. A wall need not enclose a space within the structure. An ultrasound transmitter (transducer) on the surface of the wall may be used to excite a wave that propagates in a lateral direction. This will be referred to as lateral transmission. An ultrasound receiver on the surface may be used to detect a wave signal due to a wave that propagates in a lateral direction through the wall underneath the receiver. Such a laterally propagating wave will be said to “emerge laterally” or “enter laterally” out of or into an area of the surface adjacent the receiver, without implication that the wave emerges or enters transversely, i.e. in a direction transverse to the surface. An ultrasound transceiver is both an ultrasound transmitter and an ultrasound receiver.
When the structure is a pipeline, a detection arrangement may comprise rings of ultrasound transducers and receivers around the circumference of a pipe, with an axial section of the pipe between the transducers and the receivers. In this case the delay between transmission from the transducers and reception of a reflection at the receivers in the ring provides information about the distance between the ring and a defect in the pipe and the circumferential position of the defect.
Similarly for any structure other than a pipeline, ultrasound transmission between transmitters and receivers at different positions on the wall of the structure may be used. In this case monitoring may be performed by comparing measured transmission with a baseline signal obtained from the structure without defects for each of a number of combinations of transmitters and receivers. Relative calibration of the monitoring signal and the baseline signal may be needed before the comparison to account for things like changes of transfer coefficients between ultrasound in the transducers and ultrasound in the structure.
Beyond mere detection of defects, ultrasound can be used for more detailed analysis of defects. The angle dependence of reflection may be informative about defect geometry. The magnitude of ultrasound reflections grows with defect size. For monitoring purposes it may be important to estimate the size of the defect. Intervention to repair or replace the structure may only be needed once defects exceed a threshold size. In some cases a monitoring system may be installed only after a defect has been found, in order to detect when it develops to a size that requires intervention. Therefore it is desirable for monitoring to determine the magnitude.
However, use of reflection magnitude measurements requires calibration. Apart from defect size, reflection magnitude also depends on other factors, such as transfer coefficients between ultrasound in the transducers and ultrasound in the structure, which may depend on the way in which the transducers are attached and other factors like temperature.
U.S. Pat. No. 4,255,798 discloses inspection of bore holes by means of ultrasound. In particular the quality of the bond of cement around a pipe in the bore hole is determined. A normalized energy bond signal is obtained by taking a quotient of the amplitude of reflections due to reverberation between the inner and outer casing and the amplitude of a casing reflection. In one embodiment the reverberation segment of the reflections is integrated over a particular time span to determine the energy therein. A normalized bond value, representative of the quality of the cement bond is obtained by dividing the integrated reverberation integral by the integrated casing signal.